防晒是一年四季都不能偷懒的护肤步骤，更是夏日护肤的重中之重。相信许多小仙女都知道涂防晒霜的必要性，但你真的对防晒霜足够了解吗？你知道UVA和UVB的区别吗？选择防晒霜应该注意哪些指标？

UVB rays

中波紫外线

“People think that SPF equals everything,” says Dr. Roopal Kundu, a dermatologist at Northwestern University Feinberg School of Medicine. And it does count for a lot: the sun protection factor measures a sunscreen’s ability to filter UVB rays, which are related to sunburn and skin cancer. But SPF only measures UVB rays; it doesn’t tell you anything about protection from UVA rays, Kundu says.

西北大学费因伯格医学院皮肤科医生昆都称：“人们认为防晒系数代表一切。”防晒系数的确很重要：它衡量的是防晒霜过滤中波紫外线的能力，中波紫外线与晒伤和皮肤癌有关。但防晒系数仅表示过滤中波紫外线的能力，它并不代表任何防护长波紫外线的能力。

UVA rays

长波紫外线

The most misunderstood part of sunscreen is UVA. “UVA is around every day; it can penetrate through window glass,” she says. Like UVB, it’s also related to an increased risk of skin cancer, but unlike UVB, it’s not filtered by the ozone at all, Kundu says. UVA doesn’t cause sunburn, but “it really leads to darkening and aging, because it penetrates deeper into the skin and has more influence in the collagen.”

人们对防晒霜最大的误解是长波紫外线。昆都表示，“长波紫外线随时都围绕着我们，它可以穿透窗户玻璃。”和中波紫外线一样，长波紫外线也与增加患皮肤癌的风险有关，但与中波紫外线不同的是，长波紫外线完全没有经过臭氧层的过滤。长波紫外线不会导致晒伤，“但会让皮肤变黑老化，因为它能更深入皮肤，对胶原蛋白产生更大影响”。

There’s only one way to tell whether your sunscreen offers UVA coverage: the words “broad spectrum.” Without the words “broad spectrum” your sunscreen likely doesn’t offer coverage from UVA, Kundu says.

只有一种方法可以判断你的防晒霜是否能够阻挡长波紫外线：那就是“广谱防晒”。昆都称，如果没有“广谱防晒”这个词，你的防晒霜可能无法抵挡长波紫外线。

Most active ingredients in sunscreen shield against UVB, but far fewer have UVA coverage, and only a handful offer both.

防晒霜中的大部分活性成分对中波紫外线具有屏蔽作用，但能屏蔽长波紫外线的成分却少得多，而只有少数几种成分能同时屏蔽两种紫外线。

Personally, Kundu uses an SPF 30 sunscreen with the active ingredient zinc oxide, a natural sunscreen ingredient that physically—instead of chemically—blocks rays. Zinc oxide protects against both UVA and UVB rays.

昆都个人使用的是含有活性成分氧化锌、防晒系数30的防晒霜，氧化锌是一种天然防晒成分，通过物理方式而非化学方式阻挡紫外线。氧化锌能同时阻挡长波紫外线和中波紫外线。

SPF numbers

防晒系数值

防晒系数（Sun Protection Factor）是指在涂有防晒剂防护的皮肤上产生最小红斑所需能量，与未加任何防护的皮肤上产生相同程度红斑所需能量之比值，简单说来，它就是皮肤抵挡紫外线的时间倍数。

An SPF of 30 means that technically, you could be out in the sun 30 times longer before you get sunburned than you would be able to if you went out without sunscreen, as long as you keep reapplying it appropriately.

防晒系数30意味着，理论上你在户外被太阳晒伤的时间是你没涂防晒被晒伤的时间的30倍，前提是你一直适当补涂。

例如：在不涂防晒产品的情况下，在阳光下停留20分钟后，你的皮肤会稍稍变成淡红色，则防晒系数30的产品可保护你达10小时之久。

20（分钟）*30（SPF）=600分钟（10小时）

防晒系数并不是直接反应对紫外线的抵挡能力，而且还不是一个线性关系的指标，所以防晒系数并非越高越好。相反，防晒系数越高的防晒霜使用起来越油腻、厚重、不舒适。

According to Kundu, SPF 15 filters about 93% of UVB rays; SPF 30 filters about 97% of UVB rays; and SPF 50 filters about 98% of UVB rays. The difference between SPF 30 and SPF 50 is only a 1% filtering improvement, she says, and since SPF 30 is readily available at many different price points, that’s the one many dermatologists recommend.

据昆都称，防晒系数15可过滤约93%的中波紫外线，防晒系数30可过滤约97%的中波紫外线，而防晒系数50可过滤约98%的中波紫外线。防晒系数30和50之间的防护能力仅相差1%。昆都称，因为防晒系数30的防晒霜有许多价格可供选择，所以受到许多皮肤科医生推荐。

How much you need

你需要什么样的防晒霜

Bottom line: Typical adults should buy a water-resistant, broad-spectrum SPF 30 sunscreen, reapply every two hours and use the right amount.

底线：普通成年人应该购买一款防水、广谱防晒系数30的防晒霜，每两个小时适当补涂一次。

这里需要说明的是，长波紫外线防护能力评价指标目前为止还没有全球统一标准。欧美现在更多采用的是Broad Spectrum SPF，也就是代表广谱防晒，表示产品既能防护中波紫外线也能防护长波紫外线。

但如果你购买的是日系防晒霜，需要注意的是PA值。PA（Protection UVA）是1996年日本化妆品工业联合会公布的“UVA防止效果测定法标准”，指的是防止长波紫外线的有效程度指标，是日系防晒中最广被采用的标准，防御效果被区分为三级，即PA+、PA++、PA+++，2013年又加入PA++++。